Field of the Invention:
The present invention generally relates to an imaging system for two-dimensional (2D) and three-dimensional (3D) imaging through highly scattering media in oil reservoir applications. In particular, this imaging system uses adaptive optics along with two/multi-photon microscopy in order to enhance an image quality through scattering media, observe additives (i.e. tracers, nanoparticles, and fluids) contained within reservoir rock or other scattering media, and to detect features and characteristics of reservoir rock and fluids.
Description of the Related Art:
Exploratory imaging of regions for oil and gas reserves is important in order to properly identify and extract hydrocarbons from the ground. A petroleum or oil and gas reservoir is a subsurface pool of hydrocarbons contained in porous rock formations. Rock formations trap the naturally occurring hydrocarbons and exploration methods are used to remove these hydrocarbons for further processing. Visualizing oil/water interfaces, nanoparticles, and nanoparticle mobility within reservoir rock at the sub-micron scale is a critical step toward understanding fluid and nanoparticle transport and developing appropriate models of this mobility at the reservoir scale. The objects of interest are inherently embedded below the rock surface and within the optically scattering pore structure. Many microscopy techniques have been used to image subsurface particles before, but there is a long standing problem in optical microscopy where when imaging objects of interest through scattering media, the objects of interest routinely embedded within scattering media or behind thick surfaces become blurred when they are imaged because the scattering of light obscures and distorts the image. Visualizing oil/water interfaces, nanoparticles, and nanoparticle mobility within and not simply at the surface of reservoir rock on the sub-micron scale is a critical step toward understanding fluid and nanoparticle transport and developing appropriate models of this mobility in the reservoir scale. When this transport is understood more fully it can be used to study how best to deliver materials and recover them in order to extract hydrocarbons efficiently.
Imaging fluid flow deep inside rock samples provides the most accurate representation of porous media properties. However, scattering results from imaging below the surface of the rock because of index mismatch between the solid rock phase and the fluid-filled pores. The medium is thus optically heterogeneous leading to a large amount of light scattering. The depth at which the scattering background fully obscures the image is called the transport-mean-free path (L*).
For example, in water-filled limestone, L* is on the order of 50-60 um. Therefore traditional techniques are limited to imaging the surface or near-surface flow of the liquid. However, this invention overcomes the limited ability to image the near-surface flow of liquid through Super-penetration Multi-Photon Microscopy (SP-MPM), which uses a spatial light modulator (SLM) to optimize the phase of the coherent light focused on the sample, compensating for scattering and thereby enhancing the two-photon signal. This optimization can be done at any depth, allowing for imaging deep within the sample.
Another problem associated with fluorescent microscopy is that many sub-micron fluorescent particles embedded either in an oil/water mixture or within rock pores yield few photons and are therefore difficult to image and characterize. New sophisticated techniques in two/multi-photon microscopy have recently been developed for semiconductor and biological imaging and provide the capability to image deeper within highly scattering media. One example of two-photon fluorescence microscopy (2 PM) provides imaging of sub-cortical structures at a spectral excitation window of 1,700 nm in a mouse-brain. The goal in such applications is to create an imaging method to counteract wave-front distortions caused by aberration and random scattering. However this imaging method is limited in its field of view and is good for smaller imaging applications.
The terms two-photon and multi-photon refer to the absorption of two or more photons of infrared light that are used to excite fluorescence as opposed to the use of single photon absorption in the visible wavelength range, where the use of infrared light provides a means to reduce scattering. Until now, however, these new techniques have not been applied to porous reservoir rock, oil/water interfaces, nanoparticles, tracers contained in produced fluids or in reservoir rock pieces. Furthermore, these techniques do not include adaptive optics to correct for image aberrations due to scattering interfaces either away from or in the field-of-view (FOV).
Traditionally, Computed Tomography (CT) and Nuclear Magnetic Resonance (NMR) techniques are used to characterize and image reservoir rock at the millimeter scale or at the micrometer scale with micro-CT both with and without fluids. Typically, however, when seeking information about dynamic systems (i.e. fluids, tracers, surfactants, nanoparticles, polymers, etc.) in real reservoir rock these techniques do not offer the necessary spatial or temporal resolution required. Other techniques such as optical microscopy (i.e. epifluorescence, laser scanning confocal microscopy, and total internal reflection microscopy) offer better spatial and temporal resolution but these techniques are greatly affected by noise due specifically to optical scattering thus they are mostly limited to use with transparent, solid and smooth surfaces. Additionally, of these optical techniques, only confocal microscopy is able to image in 3D.
In general, confocal microscopy uses laser light and a polished thin section of rock chip that is vacuum-pressure impregnated with fluorescing epoxy. Scans of the rock chip measure reflected or fluorescent light intensity in regularly spaced planes. 3D pore geometries are analyzed after the images are stacked using 3D visualization software. Many devices employ a multipoint statistical model in order to image the subsurface region and employ measured reflection data and noise reduction algorithms to identify noise data in the retrieved set of images, thereby creating a 3D model of the pore structure.
Laser scanning confocal microscopy (LSCM) is widely used in the biomedical community and has proved useful for imaging the pore space of reservoir rocks by filling the pores of thin sections with cured fluorescently tagged epoxy. Confocal microscopy systems are commercially available through most microscope vendors and take a series of 2D fluorescent images at multiple depths that are reconstructed into a 3D rendering of pore volume. For reservoir rocks, the pores of thin sections (<50 microns) and thick section (<250 microns) are filled with fluorescently tagged epoxy under vacuum and then cured to lock the bright fluorescent tags within the pore space. For thin sections (<50 microns) the epoxy can be imaged with the rock matrix intact but for thicker sections (<250 microns) the rock matrix must be removed through chemical methods and the epoxy cast is then imaged alone. While this technique is typically used with mobile fluids in the biomedical community, these fluids are transparent which would not be the case for crude oil samples and imaging within reservoir rock.
Traditional optical microscopy techniques fail to image deep into strongly scattered reservoir rock in the presence of oil-water mixtures. These techniques also fail to explore the dynamic motion of nanoparticles, tracers and fluid-fluid interfaces, due largely to the strong optical scattering of the light encountered at all interfaces of differing optical index, including, but not limited to variations in the material, variations in the fluids, variations in the pore walls and debris within any fluids. In addition, LSCM, which can image up to 50 microns within a rock matrix, requires that the pores be filled and cured with fluorescently tagged epoxy which renders the segment of rock useless for fluid flow purposes.
A well-known strategy to counteract aberrations makes use of adaptive optics (AO), as borrowed from astronomical imaging. Adaptive optics consists of inserting an element in the imaging optics, such as a deformable mirror (DM), which imparts inverse aberrations to the imaged light, compensating for the aberrations induced by the sample or the microscope system itself. While the optical problem of astronomy is similar to microscopy, the details of the system are different: astronomical imaging typically involves sparse, stationary signals through a dynamic medium such as turbulent atmosphere, as opposed to imaging multiple moving beads through the static rock pores.
Placement of the SLM is very important in this kind of microscopy because it can lead to further benefits in media characterization. Some inventions include that the continuous mirror or SLM is placed near the objective and not explicitly conjugated to the aperture. One drawback for placing the optical element out of the pupil plane for out-of-pupil conjugation is that even though it produces larger fields of view, this design reduces the number of actuators across the beam. The actuators act to produce a shape that corrects for the wave-front distortions. If only a few actuators are used, the shape is limited and cannot be corrected properly.
More actuators allow for better distortion correction. Another way to combat the distortion problem is to place the wave-front converting element conjugate to the pupil in order to correct the aberrations. The wave-front converting element can be used to correct for aberrations. Specifically a wave-front converting element in a laser scanning microscope (LSM) is not placed in a plane conjugate with the pupil in order to improve off-axis performance, so that loss of light can be minimized and the apparatus can be simplified. Some inventions include that the continuous mirror or Spatial Light Modulator (SLM) is placed near the objective and not explicitly conjugated to the aperture.
Specifically, adaptive optics in the form of a Kilo-DM wave-front corrector from Boston Micro-machines is also currently being used in the industry, which can be programmed to provide for specific optical corrections. The adaptive optics reconstruct a complete diffraction limited wide-field wave front in order to correct the image aberrations. Also, combining high and low resolution data using laser scanning fluorescence microscopy (LSFM) provides further advantages in imaging subsurface material. By combining high resolution LSFM scans with CT scans, larger volumes of rocks are captured to create a fuller picture of the porous material.
Some imaging devices provide for excitation in a target material by absorbing two photons. Fluorophores are excited by a conventional laser operating at twice the single-photon excitation wavelength. For power efficiency, a pulsed laser is used instead of a continuous wave laser. A dichroic mirror filters the fluorescence onto a photomultiplier tube or alternative fluorescent detector, and scanning mirrors sweep the focal spot across the sample (beam-scanning). Some implementations keep the focal spot stationary and instead use motorized stages to sweep the sample through the focal spot (sample-scanning). The fluorescence at each transverse position is recorded and processed to form the image. The main benefit of adaptive optics is the correction of aberrations that derive from the heterogeneous refractive index structure of the rock that is being imaged. The refractions from optical index variations lead to aberrations that affect the resolution and contrast of the image. These aberrations vary among different rock structures and so correcting them in real time is important for different rock formations. Furthermore, imaging at different depths tends to provide more scattering and aberrations and the techniques described herein go to enhancing these images to track and image fluid, tracer, and nanoparticle flow.
There is a need to develop systems and methods for using two/multi-photon fluorescence microscopy in conjunction with adaptive optics including deformable mirrors for enhanced imaging and detection capabilities in reservoir media. There is also a need to use advanced fluorescence techniques to allow for super-penetration imaging in conjunction with adaptive optics to compensate for aberrations both in and out of the field of interest beyond the current technologies in the field of confocal microscopy. The invention enables imaging of dynamic and stationary nanoparticles, surfactants, fluid-fluid interfaces and tracers which can be used to study properties such as diffusion, mobility, adhesion, stickiness, and wettability within the 3D pore structure of cores and thin sections. Furthermore, larger volumes of fluid will be sampled thus improving the detection of tracers and nano-particles at the well-site.